People who got bigger from weight training didn't necessarily have bigger changes in their muscle cells than those who didn't grow as much — so these classic cell measurements can't explain why some people build muscle faster than others.

We analyzed the available evidence and found that changes in muscle cells alone don’t explain why some people build muscle faster than others. In studies comparing individuals who gained more muscle from weight training to those who gained less, the differences in muscle cell changes weren’t large enough to account for the variation in results [1]. This suggests that what happens inside the muscle cells — like increases in protein or fiber size — isn’t the full story behind why one person grows faster than another. What we’ve found so far points to other factors possibly playing a bigger role, even though we haven’t looked at them directly in this set of studies. The classic measurements used to track muscle growth — such as fiber thickness or satellite cell activity — didn’t consistently match up with how much muscle people actually gained. That means if you’re trying to understand why someone responds better to training, looking only at their muscle cells might miss the bigger picture. This doesn’t mean muscle cell changes aren’t important — they’re still part of the process. But they don’t seem to be the main reason for differences in how quickly people build muscle. Other things, like how the body signals growth, how well nutrients are used, or how the nervous system activates muscles, could be contributing. We haven’t reviewed those factors here, so we can’t say how much they matter. The evidence we’ve reviewed so far doesn’t give us a clear answer about what does drive those differences — only that muscle cell changes alone don’t explain them. If you’re training and wondering why your progress doesn’t match someone else’s, it’s not necessarily because your muscles aren’t changing enough. The reasons might be deeper — and still hidden from view.